[The present disclosure relates to a display unit including a light transmissive film referred to as a luminance enhancement film.
In these years, Cathode Ray Tubes (CRT) that were a mainstream of display units in the past have been replaced with liquid crystal displays, due to advantages such as the low electrical power consumption and the space-saving, and the low cost and the like.
There are several types of the liquid crystal displays when categorized by, for example, illumination methods in displaying images. As a representative example, a transmissive display unit that displays images by utilizing a light source arranged behind a liquid crystal display panel is cited.
By the way, in such a display unit, it is extremely important to decrease the electrical power consumption and increase the display luminance in order to increase the commercial value of the display unit. Thus, it has been strongly aspired that the gain of the optical system provided between the liquid crystal display panel and the light source is increased, while the electrical power consumption of the light source is kept low as much as possible.
For example, in Japanese Patent No. 3158555, a measure of providing a light transmissive film referred to as the luminance enhancement film between a liquid crystal display panel and a light source is disclosed. A description will be hereinafter specifically given of the light transmissive film by using FIG. 24.
FIG. 24 shows a schematic structure of a transmissive display unit 100 including the foregoing light transmissive film. The display unit 100 includes a liquid crystal display panel 110, a first polarizer 120A and a second polarizer 120B that sandwich the liquid crystal display panel 110, an lighting device 130 arranged behind the first polarizer 120A, and a drive circuit (not shown) displaying an image by driving the liquid crystal display panel 110. The front face of the second polarizer 120B is oriented to the observer side (not shown).
The first polarizer 120A and the second polarizer 120B are respectively arranged so that their polarizing axes a and b are directed differently from each other by 90 degrees. Thereby, emitting light L from the lighting device 130 is transmitted through the liquid crystal display panel 110 or blocked. Further, in general, the second polarizer 120B arranged on the light emitting side is arranged so that the polarizing axis b is in the vertical direction in order to allow light passing through the second polarizer 120B to be viewed with the use of a polarized sunglass. Therefore, the polarizing axis a of the first polarizer 120A arranged on the light incidence side is in the horizontal direction.
The lighting device 130 has, for example, a plurality of linear light sources 131 extending in the horizontal direction. On the liquid crystal display panel 110 side of the linear light sources 131, a diffusion sheet 132 and a light transmissive film 133 are arranged sequentially from the linear light source 131 side. Meanwhile, a reflective sheet 134 is arranged behind the linear light source 131.
The light transmissive film 133 has a plurality of prisms 133A in the shape of a triangle pole on the face (front face) on the light emitting side. The respective prisms 133A extend in the horizontal direction and are arranged in parallel in the vertical direction. Thereby, out of light emitted from the linear light sources 131, the light mainly divergent in the vertical direction is raised in the direction orthogonal to the liquid crystal display panel 110 (front face direction) to be collect.
In the display unit 100, the emitting light L from the linear light sources 131 directly enters the diffusion sheet 132, or is reflected by the reflective sheet 134 and then enters the diffusion sheet 132. The emitting light L is evenly diffused by the diffusion sheet 132, is collected in the light transmissive film 133, and is emitted toward the first polarizer 120A. Of the light entering the first polarizer 120A, a polarization component in parallel with the polarizing axis a passes through the first polarizer 120A. The light passing through the first polarizer 120A passes through the second polarizer 120B to the observer side according to a voltage size applied to each pixel by the not-shown drive circuit.
As described above, by arranging the light transmissive film 133 between the first polarizer 120A and the diffusion sheet 132, the emitting light L from the lighting device 130 can be effectively caused to be incident on the liquid crystal display panel 110. As a result, the display luminance is able to be improved.
However, in the display unit 100 shown in FIG. 24, the light passing through the light transmissive film 133 is non-polarized light, and thus about half of the transmitted light (polarization component in the direction crossing the polarizing axis a) is absorbed into the first polarizer 120A. Therefore, only by providing the light transmissive film 133 between the liquid crystal display panel 110 and the light source, use efficiency of the illuminating light from the linear light sources 131 is not improved much, and thus the display luminance is not able to be improved sufficiently.
Thus, it is conceivable that a reflective polarizer that transmits a polarization component in parallel with the polarizing axis a (polarization component in the horizontal direction) and reflects a polarization component in the direction crossing the polarizing axis is arranged between the light transmissive film 133 and the first polarizer 120A. However, the reflective polarizer has a sandwich structure in which, for example, a multilayer film alternately stacking thin films having a refractive index different from each other is sandwiched between a pair of diffusion films. In general, such as reflective polarizer is expensive. Therefore, in the case where the reflective polarizer is used, the cost of the display unit 100 is high. Further, the polarization component in the direction crossing the polarizing axis a is somewhat leaked from the reflective polarizer and is absorbed into the first polarizer 120A. Therefore, there is room for improvement with regard to the cost, the light use efficiency, and the display luminance.
Then, to improve the light use efficiency and the display luminance at low cost, for example, it is conceivable that the light transmissive film 133 is provided with refractive index anisotropy. Specifically, instead of the light transmissive film 133, a light transmissive film 233 that is made of a semicrystalline or crystalline resin in which the refractive index in the drawing direction is larger than the refractive index in the direction orthogonal to the drawing direction, and that is provided with refractive index anisotropy by being stretched in the extending direction of the prism 133A is arranged, as shown in FIG. 25. However, the polarizing axis of the light transmissive film 233 is in the vertical direction, and thus the polarizing axis is orthogonal to the polarizing axis a of the first polarizer 120A. Accordingly, the display luminance is significantly lowered.
Therefore, it is conceivable that the first polarizer 120A and the second polarizer 120B are rearranged so that the polarizing axis a is in parallel with the polarizing axis of the light transmissive film 233, as shown in FIG. 26, or the light transmissive film 233 is rearranged so that the polarizing axis of the light transmissive film 233 is in parallel with the polarizing axis a, as shown in FIG. 27. However, in the former case, the orientation of the polarizing axis b of the second polarizer 120B becomes in the horizontal direction, and thus the polarization component of light passing through the second polarizer 120B also becomes in the horizontal direction. Accordingly, it is difficult to view the light passing through the second polarizer 120B with the use of a general polarized sunglass. In the latter case, since the light transmissive film 233 functions to collect, out of illuminating light from the linear light sources 131, the light by raising the light mainly divergent in the horizontal direction in the direction orthogonal to the liquid crystal display panel 110 (front face direction), the view angle in the horizontal direction is narrower than the view angle in the vertical direction.